In a high-frequency inverter, there generates a switching loss, a large surge voltage, and a large surge current, with an increase in frequency.
The switching loss is caused by a phase deviation between a voltage and a current upon a switching operation. When a switch is turned on, a current flows with a voltage being applied. As a result of the current flows with the voltage being applied, the product of the current and the voltage becomes a power loss.
The surge voltage and the surge current are generated, when a switch is abruptly turned off while a current steadily flows. That is to say, when ΔA/ΔT or ΔV/ΔT is large, a large surge voltage and surge current are likely to occur.
These switching loss, surge voltage, and surge current result in the loss of energy, and sometimes damage a switch. Thus, various soft switching circuits have been developed and researched, in order to suppress the switching loss, surge voltage, and surge current.
These soft switching circuits are roughly classified into zero-voltage switching (ZVS) circuits and zero-current switching (ZCS) circuits.
In the zero-voltage switching circuits, a current flows after a voltage becomes zero upon turning on/off a switch. Since the voltage is zero, there is no power loss upon a switching operation.
On the other hand, in the zero-current switching circuits, a voltage is applied after a current becomes zero upon turning on/off a switch. Also in this type, there is no power loss upon a switching operation.
Referring to FIGS. 6 and 7, a conventional zero-voltage switching high-frequency inverter is described below.
FIG. 6 shows a structure of a conventional series resonance half-bridge ZVS high-frequency inverter with two switches.
In the conventional series resonance half-bridge ZVS high-frequency inverter 10 with two switches, a first switch S1 and a second switch S2 are connected in series between power sources Ed. A first capacitor C1 is connected in parallel to the first switch S1, and a second capacitor C2 is connected in parallel to the second switch S2 A third capacitor C3, an impedance element R, and an inductor element L are connected in series between a connecting path connecting the first switch S1 and the second switch S2 and one end of the power sources Ed.
By connecting the first capacitor C1 and the second capacitor C2 in parallel to the first switch S1 and the second switch S2, respectively, the generation of excessive surge voltage/current can be prevented at the first switch S1 and the second switch S2, whereby a zero-voltage function can be fulfilled.
The third capacitor C3 constitutes a series resonant circuit together with the impedance element R and the inductor element L so as to fulfill an inverter function.
FIG. 7 shows a structure of a conventional series resonance full-bridge ZVS high-frequency inverter with four switches.
In the conventional series resonance full-bridge ZVS high-frequency inverter 11 with four switches, a first switch S1 and a second switch 2 are connected in series between power sources Ed, and a third switch S3 and a fourth switch S4 are connected in series between the power sources Ed. A first capacitor C1, a second capacitor C2, a third capacitor C3, and a fourth capacitor C4 are connected in parallel to the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4, respectively. A fifth capacitor C5, an impedance element R, and an inductor element L are connected in series between a connecting path connecting the first switch S1 and the second switch S2 and a connecting path connecting the third switch S3 and the fourth switch S4.
In the series resonance full-bridge ZVS high-frequency inverter 11 with four switches, the first and fourth switches S1 and S4 and the second and third switches S2 and S3 are respectively synchronized, and the switches are alternately turned on and off to generate an alternating current at the impedance element R and the inductor element L.
The first to fourth capacitors C1 to C4 prevent the generation of surge voltage/current and fulfill a zero-voltage switching function at the first to fourth switches S1 to S4. The fifth capacitor C5 constitutes a series resonant circuit together with the impedance element R and the inductor element L so as to fulfill an inverter function.
However, in the conventional series resonance half-bridge ZVS high-frequency inverter with two switches or the series resonance full-bridge ZVS high-frequency inverter with four switches, an amplitude of a current flowing through the switches and an amplitude of a current flowing through the load are equal to each other.
Thus, when a resistance of the load is small such as an induction heating load, the amplitude of a current flowing through the load becomes large, and a current of the same amplitude flows through the switches.
Under this state, although a switching loss can be reduced due to the ZVS, a conduction loss is disadvantageously increased by the current of large amplitude that flows through the switches, when the switches are on.
When the ZVS can make it possible that a current flowing through a load is large while a current flowing through switches (main switch current) is small, a significantly advantageous state can be attained in which both the switching loss and the conduction loss can be suppressed while allowing a large current to flow through the load.
Therefore, an object of the present invention is to provide a zero-voltage switching high-frequency inverter in which a larger current flows through a load while a smaller current flows through switches.
Further, in both the conventional series resonance half-bridge ZVS high-frequency inverter with two switches and the conventional series resonance full-bridge ZVS high-frequency inverter with four switches, each of the switches is provided with a capacitor, which entails a complicated circuit structure.
Needless to say, it is preferable that a structure of a circuit is simple.
Therefore, another object of the present invention is to provide a zero-voltage switching high-frequency inverter which is simple in structure.